Photoconductors made from epitaxial layers of GaAs or related semiconductor compounds have shown considerable promise as detectors for optical communications and as optoelectronic crosspoint switches. As switches, they provide over 60 dB isolation, 1 GHz bandwidth, low switching bias and subnanosecond switching time. (see Optical Engineering, Vol. 24, p.p. 220-224, 1985, "Optoelectronic Switch Matrices: Recent Developments" by R. I. MacDonald et al). As detectors in an optical communication link, they offer commensurate responsivity and bandwidth similar to the more common P-N and P-I-N photodiodes. The planar structures employed in the photoconductor fabrication allow ease of monolithic integration. A large area epitaxial photoconductive detector was reported in an article, "An epitaxial Photoconductive Detector for High Speed Optical Detection", Proc. IEDM, 1979 p.p. 634-637 by J. C. Gammel et al. The photoconductive detector reported therein has an N+, p-, n+ structure which is used under bias conditions similar to a punched-through transistor. The injection boundary conditions cause the p- epitaxial layer to behave as a photoconductor, thus providing high speed photoconductive gain.
Photoconductors can be made in interdigitated configurations to preserve the short channel lengths necessary for high optical speed and gain, and yet give relatively large active areas for ease of optical coupling and low capacitance. Thus, C. Y. Chen et al report a heterostructure interdigitated photoconductive detector in "Interdigitated Al.sub.0.48 In.sub.0.52 As/Ga.sub.0.47 In.sub.0.53 As Photoconductive Detectors", Applied Physics Letters, Vol. 44, No. 1, Jan. 1, 1984, p.p. 99-101. Their detector shows a rise time of 80 ps, a fall time of 1.2 ns and a peak responsivity seven times better than that of a commercial PIN photodiode. An article entitled "On the Responsive Behaviour of Fast Photoconductive Optical Planar and Coaxial Semiconductor Detectors" by H. Beneking, IEEE Trans. on Electron Devices, Vol ED-29, No. 9, Sept. 1982, p.p. 1431-1441, reviews fast optical detectors, which use photoconductive effects, in semiconducting channels or thin films.
The major disadvantage of most epitaxial photoconductors is their large bias current, which can be of the order of tens of mA at operating bias voltages of 10 V. The difficulty in making low-current GaAs photoconductors, for example, is that the resistivity of conventionally prepared, undoped epitaxial material is typically a few ohm-cm while the absorption length of the light to be detected is of the order of 1 .mu.m. In consequence, the sheet resistivity of epitaxial photoconductive layers suitable for efficient photoconductors is 10-30 k.OMEGA. per square.
When interdigitaed photoconductor configurations (aspect ratio of less than 10.sup.-2) are used to achieve the narrow channel length required for significant photoconductive gain (less than about 10 .mu.m) simultaneously with a large photosensitive area of about 100 .mu.m square, the channel width is correspondingly large resulting in a resistance of a few hundreds ohms, and the dark current is usually of the order of a few milliamperes. Such high currents contribute to the detector noise and also cause an undesirable shift in the output level when the device is used as an optoelectronic crosspoint switch.
To avoid this problem photoconductors fabricated directly in Cr-compensated semi-insulating gallium arsenide have been reported in "Frequency and Pulse Response of a Novel High Speed Interdigital Surface Photoconductor (IDPC)" by C. W. Slayman et al, IEEE Electron Device Letters, Vol. EDL-2, No. 5, May 1981, p.p. 112-114. Cr dopants introduce electron traps that remove free carriers. These photoconductors exhibit very low bias currents (10 .mu.A at 20 V bias). However the high density of deep traps shortens the lifetime of photogenerated carriers and the photoconductive gain is sacrificed.
U.S. Pat. No. 4,490,709, Dec. 5, 1984 Hammond et al describes an InP:Fe photoconductive device. Instead of GaAs doped with Cr in the above-referenced article by Slayman et al, Hammond et al use InP doped with Fe as semi-insulating semiconductive material. Metal contacts are directly deposited on the Fe doped Inp. Similar results to those obtained by Slayman et al are given in the patent.